Wavelength division multiplexed (WDM) communication system offer a high data transmission capacity by allowing multiple laser sources to transmit many high-speed data channels simultaneously into a single fiber, where each channel is transmitted at a unique optical frequency (or wavelength). In order to standardize the frequencies of the channels across telecommunication systems, the industry has adopted a standard which specify that the nominal optical frequency of every channel should be at an integer multiple of 100 GHz, 50 GHz, 25 GHz or even smaller spacings. The absolute frequencies accuracy must typically be within 2.5 GHz or 1.25 GHz, or even better for those systems with the highest channel densities.
Semiconductor lasers currently used in telecommunication systems do not intrinsically generate frequencies accurate or stable enough to be used alone in such a frequency grid system, whether they are narrowly or widely tunable lasers. This is caused by many reasons. First, current fabrication technologies do not permit to know with sufficient accuracy the nominal frequency of the lasers with respect with the frequency tuning signal. Second, the frequency of the laser varies significantly with operating conditions and environmental factors such as temperature. Third, even if all other parameters are kept constant, the frequency of a laser tend to drift with aging. All these factors can easily detune a laser frequency beyond the accepted limit during its lifetime.
Various means have been devised to stabilize the frequency of semiconductors to a predetermined value with a sufficient accuracy. Many of those use an optical frequency reference element which is sufficiently accurate and stable for the telecommunication applications. This reference element is used to compare the frequency of the laser with the predetermined value and generate an error signal which is fed back to the laser to correct its frequency. Once the feedback system is operational and the laser is frequency-locked, the stability of the reference is transferred to the laser.
Fabry-Perot resonators are often used in telecommunication devices in order to provide regularly-spaced frequency reference points over a broad range of frequencies. High-finesse resonators display narrow transmission peaks that can be used to accurately pinpoint specific frequencies, while between these peaks there is a zone where transmission is weak and show little variations. On the opposite, low-finesse resonators do not display sharp peaks but rather wide and flat peaks, with a transmission that varies periodically and continuously from peak to peak. Low-finesse resonators are useful when it is needed to know how far the frequency of a source is from the center of the peak by measuring the transmission of the resonator.
There are some applications where both a high finesse and low finesse resonator would be useful. For example, while locking a laser to a resonator peak, a low finesse would allow the laser to have a wide locking range, and once the laser is locked, a high-finesse would be needed to provide high locking accuracy.
Therefore, it would be advantageous to provide a single resonator simultaneously displaying the advantages of high and low finesse resonators, since high finesse peaks allow accurate frequency pinpointing and locking, and a low finesse peaks allows to extend the locking range and also allows frequency interpolation.
Other applications may require to have a device which display a more complex frequency response than a simple Fabry-Perot resonator can provide and yet have a device that generate this frequency response repeatedly over a large frequency range. It would therefore be advantageous to provide a complex frequency response filter providing a frequency response having a desired shape.